Abstract
Lean-burn gas engines have been operated worldwide in various fields such as the key hardware to the cogeneration system. Lean-burn, however, has crucial problems of unstable ignition and poor flame propagation such as misfire. Although one technical solution to this is the pre-combustion chamber method, even more complex problems could arise depending on the structure of each combustion chamber. For example, excess air ratio in the pre-combustion chamber of lean-burn gas engine is not completely uniform. The ignition method using micro-pilot fuel oil instead of spark plug as an ignition source has been studied. It has been investigated that the effect on the engine performance of above two ignition methods. Micro-pilot ignition method has following characteristics and merits compared with spark ignition method : i. Secure ignition ensured by compression ignition of fuel oil eliminates unstable ignition and misfire in the precombustion chamber. ii. Ignition energy is much larger than that of spark plug, which shortens combustion duration. This characteristic enlarges knock margin and enables higher BMEP output. iii. NOx emission depends on the amount of fuel oil in this micro-pilot ignition. NOx level is low because the amount of the fuel oil is mere about 1% of the total input fuel energy. Therefore, under the same restrictions of NOx concentration, larger pre-combustion chambers are allowed in the engine. It contributes to improve thermal efficiency. Multi-ignition and swirl are key factors to improve thermal efficiency of the engines. Multi-ignition enables to shorten the distance of flame propagation and swirl accelerates the flame propagation in the mixture. Optimizing the direction of a jet hole of pre-combustion chambers against swirl flow should be considered as an important design factor to obtain higher thermal efficiency. A single cylinder engine of a 260mm bore and a 275mm stroke has been used for combustion tests. The engine performance has been investigated at BMEP of 1.47MPa. The effect of turbulent flow to combustion of lean-mixture in the combustion chamber has been also investigated. This paper describes the engineering findings of combustion characteristics of the micro-pilot ignition method and the effect of the direction of a jet hole against the swirl flow on thermal efficiency with low-NOx emission.
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